Adhesive bonding is a well known method for joining similar and dissimilar materials. However, the advent of both metallic and nonmetallic materials which are capable of withstanding high temperatures has generated a need for high-temperature stable adhesives for bonding advanced materials. The increased use of titanium and thermally resistant composites and films for aerospace applications has led to the investigation of polyimides as base ingredients for adhesive formulations. Due to their excellent thermal stability, the linear polyimides should make good candidates. The room temperature adhesive strengths of linear polyimides are high, but unfortunately the normal thermoplastic nature of those polymers most suitable as polyimide ahesives prevents them from exhibiting good adhesive strengths at temperatures in the range of 250.degree.-350.degree. C. on such metals as steel or titanium. Modifications to improve the adhesive properties of linear polyimides have generally been made at a sacrifice to the long term thermal stability of the resins.
Two modifications which have made considerable improvements in the overall adhesive properties of linear polyimides are: (1) the preparation of the polymers from isomeric metaoriented aromatic diamines, and (2) the preparation of the polymers in certain water- or alcohol-miscible ether solvents. The addition of large amounts of aluminum (Al) powder to the resins prior to bonding is another modification that has served to improve the adhesive strengths of polyimides at high temperatures. The Al powder acts as a filler to increase the softening temperature of the polymer and also acts as a buffer to aid the polymer in matching more closely the thermal coefficient of expansion of the metal adherend.
Although high temperature strengths are improved, the addition of metal fillers to polyimides produces certain disadvantages. One drawback is the drastic increase in weight caused by the added metal which, in effect, defeats the use of a polymeric adhesive on aircraft or spacecraft for the purpose of weight-savings. Also, polyimide adhesives tend to weaken with the addition of metal fillers due to a loss in flexibility of the adhesive joint. The added metal embrittles the fully cured polymer, thus sacrificing toughness. Thus, there is a definite need in the art for an improved high temperature strength adhesive for use in aerospace applications.
It is therefore an object of the present invention to provide an improved lightweight, adhesive for joining structures adapted for exposure to high temperatures.
Another object of the present invention is a method of preparing a high temperature stable polyimide adhesive.
A further object of the present invention is a method of making an aluminum ion-containing polyimide for use as an adhesive to join metallic structures.